Tutorial 1

Thomas E. Marlin

Description

McMaster UniversitySolutions for Tutorial 1Feedback Concepts & Benefits1.1. Drawing symbols: Determine all letters that would be used to designate each ofthe following instruments on process and instrumentation (P&I) drawings.The approach for assigning symbol letters is explained in Appendix A in Marlin,2000. Much more detail is provided in ISA 5.1-1984.For example, for a level controller, the designation would be LC.i) Liquid level alarm high, LAH Set value for the alarm is NOTshown on the drawingii) Pressure indicator, PI Not used for controliii) Temperature indicator in a packed TI “in a packed bed” is notbed, relevant; the symbol isindependent of the processapplicationiv) Volume flow rate of butane in a pipe, FI Flow == Fv) Mass flow rate of hydrogen, FI The units of the flow are notindicated in the symbol.vi) Weight of a solid in a vessel,vii) Speed of rotation of a shaft, and SIviii) Mole % of propane in a gas stream AI “Analyzer” is AYou might wonder, “Where are the details?” A detailed instrumentspecification sheet is completed for every sensor. This indicates the streamconditions, physical principle, range of operation and other information. Youwill be able to purchase the instrument and design installation based on theinformation in the data sheet.Conclusion: We must use a standard set of symbols so that all engineers andplant operators understand the design.12/27/00 Copyright © 2000 by Marlin and Yip 1McMaster University1.2. ...

Informations

• Publié par: Siop
• Nombre de lectures: 42
• Langue: English

Extrait

McMaster University

12/27/00

Copyright © 2000 by Marlin and Yip

1

Solutions for

Tutorial 1

Feedback Concepts & Benefits

1.1.

Drawing symbols

: Determine all letters that would be used to designate each of

the following instruments on process and instrumentation (P&I) drawings.

For example

, for a level controller, the designation would be LC.

i)

Liquid level alarm high,

LAH

Set value for the alarm is NOT

shown on the drawing

ii)

Pressure indicator,

PI

Not used for control

iii)

Temperature indicator in a packed

bed,

TI

“in a packed bed” is not

relevant;

the

symbol

is

independent of the process

application

iv)

Volume flow rate of butane in a pipe,

FI

v)

Mass flow rate of hydrogen,

FI

Flow == F

The units of the flow are not

indicated in the symbol.

vi)

Weight of a solid in a vessel,

vii)

Speed of rotation of a shaft, and

SI

viii)

Mole % of propane in a gas stream

AI

“Analyzer” is A

The approach for assigning symbol letters is explained in Appendix A in Marlin,

2000.

Much more detail is provided in ISA 5.1-1984.

You might wonder, “Where are the details?”

A detailed instrument

specification sheet is completed for every sensor.

This indicates the stream

conditions, physical principle, range of operation and other information.

You

will be able to purchase the instrument and design installation based on the

information in the data sheet.

Conclusion: We must use a standard set of symbols so that all engineers and

plant operators understand the design.

McMaster University

12/27/00

Copyright © 2000 by Marlin and Yip

2

1.2.

Common examples of automation:

Discuss whether each of the common

systems below uses automatic

feedback

to achieve its desired performance.

a.

Boiling water on a burner in a home stove.

b.

Maintaining a temperature in an oven in a home stove.

c.

An alarm clock used to wake you for class.

Note: The question asks if automatic feedback is applied.

“Automatic” implies

the use of a computing device, such as a digital computer.

Feedback could be

applied by a person, which is generally not as reliable.

We’re smart but we get

tired.

The burner is set to a constant gas flow or electrical power, and no automatic

adjustment is applied to achieve a desired rate of boiling.

Note that the temperature is constant when the water is boiling, regardless of the

heating applied.

This is NOT due to control, but is a result of the process

principles.

The typical home oven has a temperature controller.

The automatic approach is

not complex; it applies and on/off feedback algorithm.

If the temperature is

below a set point, the furnace is turned on; if the temperature is above a set point,

the furnace is turned off.

Usually, a “dead band” is applied to prevent the heater

from switching on and off too frequently.

No automatic mechanism is applied to the alarm clock.

If the power fails, the

clock cannot recognize this and correct.

Also, if you do not awake, the clock stops

sounding the alarm after a specified time.

So, the success of the alarm depends on our participation, which we regret every

morning.

Conclusion: We apply automatic feedback control when we desire reliable

application of a consistent policy.

McMaster University

12/27/00

Copyright © 2000 by Marlin and Yip

3

1.3.

A Chemical Engineering Example:

A chemical reactor with recycle is depicted in

textbook Figure 1.8 and repeated below.

a.

Can the following variable be controlled by feedback?

Hint: determine which valves have a

causal effect on each sensor.

b.

Select the best valve to control each, if more than one valve can effect the sensor.

c.

Select a sensor principle for each of the sensors.

(

Hint

: Check the WEB site!)

i.

T4, reactor feed temperature

ii.

T1, feed temperature

iii.

F3, reactor effluent flow

iv.

L1, reactor liquid level

Figure 1.1

T4, reactor temperature

v1 Yes,

strong

This will influence the flow rate through the feed exchanger and

the ratio of fresh to recycle, which can be at different

temperatures.

v2 Yes,

strong

This will affect the flow of heating oil to the feed heat exchanger.

v3 Yes,

weak

This will affect the flow rate of both fresh and recycle feeds,

without changing the ratio.

v4 Yes,

temporary

This will change the recycle flow temporarily.

Note that the

supply of recycle material is limited that the average over time can

be no more (or less) than what remains liquid in the flash drum.

v5 no

This affects the flow out of the reactor.

v6 no

See v5 above

v7 Yes,

weak

This will affect the heat to the reactor effluent, which influences

the flow rate and temperature of the recycle.

v8 Yes,

weak

This will affect the pressure in the flash drum and thus, the

fraction of reactor effluent that is vapor.

The liquid recycles to

the reactor.

The best choice should provide a fast and strong effect on T4 and leave valves for

other important controllers.

Let’s select v2.

Because this is a reactor, we could select an RTD sensor for good accuracy, but we

need more information.

v1

Hot Oil

v2

v3

L1

v7

v5

v6

Hot Oil

F1

T1

T3

T2

F2

T4

T5

F3

T6

T8

F4

L1

v8

T7

P1

F5

F6

T9

McMaster University

12/27/00

Copyright © 2000 by Marlin and Yip

4

1.4

Economic benefits:

Discuss the economic benefits achieved by reducing the

variability (and, in some cases changing the average value) of the key controlled variable

for the situations in the following.

a.

Crude oil is distilled, and one segment of the oil is converted in a chemical reactor

to make gasoline.

The reactor can be operated over a range of temperatures; as

the temperature is increased, the octane of the gasoline increases, but the yield of

gasoline decreases because of increased by-products of lower value.

(It’s not

really

this

simple, but the description captures the essence of the challenge.)

The

customer cannot determine small changes in octane.

You are responsible for the

reactor operation.

Is there a benefit for tight temperature control of the packed

bed reactor?

How would you determine the correct temperature value?

In this situation, the customer cannot distinguish small changes from the

minimum octane when driving their automobiles.

Therefore, this small deviation

in product quality is acceptable.

However, the variability in the octane results in

a lower average yield of gasoline and a higher yield of lower valued byproducts.

Tight control of reactor temperature will reduce the variability in octane and

allow a higher average yield of valuable gasoline.

The average temperature can

be selected to achieve acceptable octane for all production within the variation.

Note that the goal here is to reduce variability and adjust the average value to

increase profit.

Octane

Time

Octane

Gasoline yield, %

Minimum

Maximum possible

yield

Average yield achieved

because of “backoff”

from limit

McMaster University

12/27/00

Copyright © 2000 by Marlin and Yip

5

b.

You are working at a company that produces large roles of paper sold to

newspaper printers.

Your client has many potential suppliers for this paper.

Your

customer can calibrate the printing machines, but after they have been calibrated,

changes to paper thickness can cause costly paper breaks in the printing machines.

Discuss the importance of variance to your customer, what your product quality

goal would be.

Is this concept different from the situation in part (a) of this

question?

In this situation, the average paper thickness is not extremely important, so long

as the customers can calibrate their machinery.

However, after you and the

customers have agreed on a thickness, essentially any variation is harmful,

because it increases the likelihood of paper breaks.

The customers lose

production time, paper, and perhaps, the workers are subject to hazardous

conditions.

If you do not supply consistent thickness, the customer will find

another supplier.

Therefore, the goal here is to retain the agreed average and reduce the

variability to the minimum achievable.

Average

number

of breaks

Paper thickness

Desired thickness